Implantable medical devices for producing a therapeutic result in a patient are well known. The implanted medical device often requires electrical power to perform its therapeutic function. This electrical power is derived from a power source. There are many kinds of powered, implantable medical devices that are powered by an external power source. It is recognized that other implantable medical devices are envisioned which also utilize energy transferred from or delivered by an external source.
By way of example, one type of powered, implantable medical device is a neurostimulation device. Neurostimulator devices, such as implantable pulse generators (hereinafter “IPGs”), are battery-powered devices that deliver therapy in the form of electrical stimulation pulses to treat symptoms and conditions, such as chronic pain, urinary incontinence, Parkinson's disease, deafness, or epilepsy, for example. IPGs deliver neurostimulation therapy via leads that include electrodes located proximate to the muscles and nerves of a patient. Treatments may require two external devices: a neurostimulator controller and a neurostimulation device charger. Neurostimulator controllers are frequently used to adjust treatment parameters, select programs, and download/upload treatment information into/from the implantable device. Neurostimulation device chargers are used to transcutaneously recharge batteries or capacitors in the implanted device.
Transcutaneous transmission of energy from an external transmitter to an internal receiver is known in the prior art. Several implantable medical devices, including an IPG, employ a replenishable power source such as a storage capacitor or a rechargeable battery. This replenishable power source can be recharged when necessary using transcutaneous energy transfer (hereinafter “TET”) from an external power source, i.e., energy is transferred non-invasively through the skin via electromagnetic communication between an external transmitter coil and an implanted receiver coil. TET involves the process of inductive coupling between two coils positioned in close proximity to each other on opposite sides of a cutaneous boundary. The external transmitter coil, composed of a plurality of wire windings, is energized by a source of alternating electrical current. This flow of electrical current in the external transmitter coil induces a corresponding current in the windings of the internal receiver coil. This resultant current can be applied to recharge the battery of the implanted medical device, or, in addition, can directly energize the IPG. Optimum transcutaneous energy transfer efficiency is achieved when the external transmitter coil is disposed on the patient's skin, directly opposite the implanted receiver coil, with a minimum separation distance between the external transmitter coil and the implanted receiver coil.
Though TET provides the advantage of non-invasive recharging of an IPG, TET is not without certain shortcomings. For example, the efficiency of transcutaneously inducing a current in the implanted coil is detrimentally affected if the external TET coil and implanted coil are not properly aligned. Though the operator of the TET device may use the visual or tactile signs of implantation to approximate the location of the IPG, precise alignment of the TET coil and charging coil is extremely difficult without the aide of an alignment indicator. Because there is no physical connection between the external TET device and the IPG to provide feedback, ascertaining whether the efficiency of energy transfer is maximized is problematic.
Even if the TET device is properly aligned with the IPG at the initiation of the charging process, the correct alignment of the devices may not endure over the period of time required for energy transfer. Energy transfer can continue for a significant period of time, ranging from several minutes to hours, before the IPG is fully recharged. During this time, it is often impracticable for the external TET coil to maintain ideal alignment with the IPG receiver coil. The patient's movement may cause the external TET coil to move, thereby misaligning the TET coil with the IPG receiver coil and reducing the efficiency of energy transfer. Therefore, it would be advantageous to provide a TET device that could indicate the real time alignment (or misalignment) of the devices and visually direct the operator toward regaining optimal alignment, thereby increasing the efficiency of energy transfer and decreasing the amount of time required for the IPG charging process.
In addition, prolonged exposure to the electromagnetic fields and heat generated by the external TET coil and the IPG can result in damage to human skin and adjacent tissues. The resulting damage generally increases with the length of exposure time. Therefore, it is desirable to limit the amount of time required to recharge the battery of an IPG using a TET device. If the devices are poorly aligned, the efficiency of transcutaneous energy transfer is reduced and the length of time required to charge the IPG is increased, thus extending the patient's exposure time to electromagnetic radiation and heat. Reducing the exposure time by improving device alignment would reduce potential tissue injury. Therefore, though existing TET devices have been generally adequate for their intended purposes, they are not entirely satisfactory in every aspect.